Process for applying a heat shielding coating system on a metallic substrate

ABSTRACT

The invention provides a process for applying a heat shielding coating system on a metallic substrate. The coating system comprises at least three individual layers selected from the group of barrier layer, hot gas corrosion protection layer, protection layer, heat barrier layer, and smoothing layer. The coating system is applied to the metallic substrate by low pressure plasma spraying in a single operation cycle. This process enables the layers to be applied in an arbitrary sequence. The process is preferably used in applying a coating system to a turbine blade, particularly a stator or a rotor blade of a stationary gas turbine or of an aircraft engine, or to another component in a stationary or aircraft turbine that is subjected to hot gas.

BACKGROUND OF THE INVENTION

[0001] The most significant progress, as far as an increase in performance of machines like turbines are concerned, can be realized by increasing the process temperature. However, such increase in process temperature can result in the fact that metallic components of the machine are stressed beyond the limits of their safe operating area such that these components will not endure an operation under these conditions for a long time without damage or at least alteration of their properties.

[0002] It is well known in the prior art to make use of coatings applied to such metallic components in order to protect them from such critical operating conditions. For example, ceramic heat shield coats are used to decrease the heat conductivity between process chamber and machine component, or metallic coats to increase the hot gas corrosion resistance of the surface of such metallic machine components. For many years, such coats are also applied by a thermal coating process; nowadays, they are state of the art.

[0003] Since in most cases one single coat is not sufficient to resist a complex stress attack, —particulary if the stress is extremely high—, preferably a coating system consisting of a plurality of different layers is applied; thereby, each layer has specific properties particularly suitable to withstand a specific stress. A typical example is to apply a stabilized zirconium dioxide layer, serving as a heat shield layer, onto a metallic layer that is resistant against hot gas corrosion, for example a MCrAlY-layer, whereby M represents a metal on the basis of cobalt, nickel or iron. Preferably, such a layer is applied directly onto the component to be protected.

[0004] Following the requirements regarding performance and life span, in the past further layers have been developed to be applied in addition to the two-layer-systems “Stabilized Zirconium Oxide/MCrAlY”. Since it can happen at high temperatures that a diffusion of important metal atoms occurs between the substrate and the MCrAlY-layer, the last named layer changes its properties in a negative sense until it cannot fulfill its function any longer. In order to prevent this side effect, an intermediate layer, located between the substrate and the MCrAlY-layer, has been developed, serving either as a diffusion barrier or as a donator of important metal atoms (designated in the following as “barrier layer”). A further intermediate layer is already used for the region between the MCrAlY-Layer and the barrier layer which reduces the oxidative attack to the MCrAlY-layer and improves the adherence to the barrier layer.

PRIOR ART

[0005] U.S. Pat. No. 5,238,753 discloses a thermal barrier coating system for high temperature superalloys that includes an intermetallic bond coating on the substrate, e.g. a metallic base body member for an aircraft jet engine turbine blade made of a Cr—Co—Fe-alloy or another alloy on the basis of CO and Ni, and a ceramic topcoat having a columnar grain structure with the columnar axis perpendicular to the surface of the coating. The intermetallic coating is preferably a nickel aluminide or a platinum aluminide, whose upper surface is oxidized during processing to form a thin layer of predominantly aluminum oxide. The ceramic topcoat is preferably zirconium oxide having from about 6 to 20 percent yttrium oxide. The ceramic topcoat is applied to the substrate by a EB-PVD method, i.e. Electron Beam Physical Vapor Deposition, whereby zirconium oxide or yttrium oxide is vaporized from a metallic body member by means of an electron beam gun.

[0006] Further methods and examples of applying a heat shield layer system onto a gas turbine blade are disclosed in U.S. Pat. Nos. 5,514,482 and 4,409,659.

[0007] In U.S. Pat. Nos. 4,321,310 and 4,321,311, heat shield layer systems are disclosed having a primer layer of the type MCrAlY between the zirconium oxide layer and the metallic substrate. As a possible method of manufacturing a heat shield layer of zirconium oxide, a PVD method is suggested, i.e. method based on physical vapor deposition.

[0008] The German Patent Document A1-197 41 961 suggests that it may be advantageous to provide for a chemical binding of the heat shield layer to the metallic primer layer in view of an increased life span and an improved adherence of the heat shield layer system to the substrate. This is realized for example by providing a thin layer of Al₂O₃. As a primer layer, as well a layer of a ternary Al—Zr—O compound may be used. The ternary Al—Zr—O compound, e.g. Al₂Zr₂O₇ is preferably used for binding a heat shield layer comprising zirconium oxide.

[0009] The heat shield layer preferably comprises a metallic substance, particularly zirconium oxide. This metal oxide is preferably alloyed with a stabilizer, e.g. yttrium oxide, to prevent a phase change at high temperatures. The zirconium oxide is alloyed preferably with 3 to 20% by weight, particularly with 8% by weight of yttrium oxide. Also other rare earth substances, like e.g. cerium oxide or scandium oxide, can be used as stabilizers for zirconium oxide.

[0010] All these layers are applied by partially very different methods, mainly in order to save costs: The barrier layers are for example galvanically applied; the hot gas corrosion protection layer e.g. by means of LPPS (Low Pressure Plasma Spraying) or HVOF (High Velocity Oxygen Fuel); the protection layer e.g. by means of PVD (Physical Vapor Deposition); and the heat shield layer e.g. by means of APS (Atmospheric Plasma Spraying) or EB-PVD (Electron Beam Physical Vapor Deposition). It is understood that all these different application methods require the provision of a huge amount of available equipment for the different technologies, resulting in partially high manufacturing costs. A particular disadvantage in connection with the EB-PVD method is the extremely high investment required for the electron beam gun, for an apparatus to provide a high-vacuum, for the high-vacuum chamber and for the partial pressure control apparatus. Moreover, the capacities of the particular methods cannot be expanded to all layers. By means of the EB-PVD method, for example, the areas of a substrate that are not directly visible during the coating operation cannot be coated at all or only insufficiently. The more multifarious the choice of the different layers is made, the more complex the variety of the coating technologies will get.

OBJECTS OF THE INVENTION

[0011] It is an object of the present invention to replace the plurality of different coating methods, that have been required for applying the different layers, by a single coating method.

SUMMARY OF THE INVENTION

[0012] To meet this an other objects, the present invention provides a process for applying a heat shielding coating system on a metallic substrate. The coating system comprises at least three individual layers selected from the following group of layers:

[0013] Barrier layer;

[0014] Hot gas corrosion protection layer;

[0015] Protection layer;

[0016] Heat barrier layer;

[0017] Smoothing layer;

[0018] The coating system is applied to the metallic substrate by low pressure plasma spraying in a single operation cycle.

[0019] In the following, the low pressure plasma spraying method (LPPS) is subdivided into the LPPS-Thick Film Method (conventional LPPS) and the LPPS-Thin Film Method (new LPPS according to U.S. Pat. No. 5,853,815).

[0020] Up to now, the simplification of the manufacturing process reached by the present invention was not possible because the thickness of the particular layers was different from layer to layer, typically a few micrometers in the case of the intermediate layers up to a few millimeters in the case of the heat shield layers. By using the processes know in the past, either only a thin layer or only a thick layer could be applied to a substrate, due both to technological and economical reasons. The U.S. Pat. No. 5,853,815 discloses a LPPS-Thin Film Method that is fundamentally suitable to apply a heat shielding layer system of the kind referred to onto a metallic substrate.

[0021] In this LPPS-Thin Film Method, a plasma torch is created in an atmosphere of particularly low pressure. Compared to older LPPS-Thick Film Methods, a plasma torch results that is considerably enlarged in transversal direction and has a de-focusing effect on a powder jet injected into the plasma torch by means of a conveying gas. Within a period of time, considered short in the field of thermal coating processes, a great area can be treated with the plasma jet containing the dispersed coating material. By using such a LPPS-Thin Film Method, in which a plasma jet with a length of up to 2.5 meters is used, very thin an uniform layers of coating material can be applied to a substrate.

[0022] In order to develop a coating system having a well defined density, the coating system has to be built-up with a plurality of individual coat applications. A suitable coating material consists of a mixture of powder particles, the mean particle diameter preferably being less than 50 μm. Each and every individual particle whose diameter is not substantially greater than the afore mentioned mean diameter is partly or fully molten in the plasma jet, with the result that, upon the molten particles hitting the surface of a substrate, a coating layer is created having a well defined density and thickness. The microscopic structure of the applied layer is adjustable, as far as its density and porosity, respectively, is concerned, by suitably selecting the spraying and powder parameters.

[0023] The application of the LPPS coating process for the creation of the entire layer system unveils for the first time the possibility to create both thin and thick layers without the need of changing the coating technology and/or equipment, as it has been required up to now.

[0024] The layer system as a whole can be heat treated after having been applied to a substrate.

[0025] The preferred parameters of the layers coming into consideration are summed up in the following table. TABLE LAYER MATERIAL THICKNESS OF LAYER Barrier Layer Metallic, particularly 1 to 20 μm, preferably 8 metal alloy, preferably to 12 μm NiAl- or NiCr-Alloy Hot Gas Corrosion Protec- Metallic, particularly 50 to 500 μm, preferably tion Layer MCrAlY-Alloy (whereby M 100 to 300 μm is Fe, Co or Ni), or Metal Aluminid Protection Layer Aluminum Oxide or Ternary 1 to 20 μm, preferably 8 Al-Zr-O-Alloy to 12 μm Heat Shield Layer Oxide ceramic substance, 100 to 2000 μm, preferably particularly Zirconium 150 to 500 μm oxide containing sub- stance, and stabilizer, particularly rare earth oxides, preferably Yt- trium oxide or Cerium ox- ide Smoothing Layer Oxide ceramic substance, 1 to 50 μm, preferably 10 particularly Zirconium to 30 μm oxide containing sub- stance, and stabilizer, particularly rare earth oxides, preferably Yt- trium oxide or Cerium ox- ide

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In the following, embodiments of the invention will be further described, with reference to the accompanying drawings, in which:

[0027]FIG. 1 shows a schematic sectional view of a heat shield layer system manufactured according to the embodiment of the invention described herein after; and FIG. 2 shows a microscopic picture of a heat shield layer system manufactured according to that embodiment, in which the different layer structures are evident.

EXAMPLE 1

[0028] In the present embodiment for manufacturing a heat shield layer system by means of the LPPS coating process according to the invention, first, a barrier layer is applied under thin film conditions. Thereafter, a primer layer and a hot gas corrosion protection layer is deposited under thick film conditions. Then, a coat of a protection layer is applied under thin film conditions; and finally, a smoothing layer is applied under thin film conditions

[0029] The resultant heat shield coating system comprises a structure as seen in FIG. 1. The reference numerals have the following meaning:

[0030] 1 The substrate (e.g. Ni- or Co-Alloy);

[0031] 2 Metallic barrier layer (e.g. NiAl- or NiCr-Alloy—1 to 20 μm);

[0032] 3 Metallic hot gas corrosion protection layer (e.g. MCrAlY-Alloy 50 to 500 μm);

[0033] 4 Oxide ceramic protection layer (e.g. Al₂O₃—1 to 20 μm);

[0034] 5 Oxide ceramic heat shield layer (e.g. ZrO₂-8%Y₂O₃—100 to 2000 μm);

[0035] 6 Oxide ceramic smoothing layer (e.g. ZrO₂-8%Y₂O₃—1 to 50 μm).

[0036] It is understood that the above described embodiment is not to be considered as limiting at all, but that other layer systems different than the one described herein above can be applied, of course within the scope of the appended claims. Particularly, the present invention provides for applying the individual layers in every arbitrary sequence.

EXAMPLE 2

[0037] Following the process described in Example 1 herein above, the layer sequence illustrated in FIG. 2 has been manufactured. The parameters are as follows: 10 Substrate Superalloy Inconel 718, 3 mm thick 11 Barrier Layer AMDRY (Ni80% Cr), 13 μm thick 12 Hot Gas Corrosion AMDRY 9951 (Co 32% Ni 21% CR 8% Al Protection Layer 0.5% Y), 137 μm thick 13 Protection Layer Metco 105 (99.5% Al₂O₃) 9 μm thick 14 Heat Shield Layer Metco 204, ZrO₂-8% Y₂O₃, 360 μm thick 15 Smoothing Layer Metco 204, ZrO₂-8% Y₂O₃, 15 μm thick. 

What is claimed is:
 1. A process for applying a heat shielding coating system on a metallic substrate, said coating system comprising at least three individual layers selected from the following group of layers: Barrier layer; Hot gas corrosion protection layer; Protection layer; Heat barrier layer; Smoothing layer; said coating system being applied to said metallic substrate by low pressure plasma spraying in a single operation cycle.
 2. A process according to claim 1 in which said coating system is applied to said metallic substrate in a single operation cycle without any interruption.
 3. A process according to claim 1 in which said coating system is heat treated as a whole after having been applied to said substrate.
 4. A process according to claim 1 in which a barrier layer with a thickness of 1 to 20 μm, preferably of 8 to 12 μm, is applied to said metallic substrate.
 5. A process according to claim 4 in which a metallic barrier layer, particularly consisting of a metal alloy, preferably a NiAl- or a NiCr-alloy, is applied to said metallic substrate.
 6. A process according to claim 1 in which a hot gas corrosion protection layer with a thickness of 50 to 500 μm, preferably of 100 to 300 μm, is applied to said metallic substrate.
 7. A process according to claim 1 in which a metallic hot gas corrosion protection layer, particularly consisting of a MCrAlY-alloy, whereby M is Fe, Co or Ni, is applied to said metallic substrate.
 8. A process according to claim 1 in which a protection layer with a thickness of 1 to 20 μm, preferably of 8 to 12 μm, is applied to said metallic substrate.
 9. A process according to claim 8 in which a protection layer consisting of an aluminum oxide or a ternary Al—Zr—O-compound is applied to said metallic substrate.
 10. A process according to claim 1 in which a heat barrier layer with a thickness of 100 to 2000 μm, preferably of 150 to 500 μm, is applied to said metallic substrate.
 11. A process according to claim 10 in which a heat barrier layer, consisting of an oxide ceramic substance, particularly a zirconium oxide containing substance, and a stabilizer, particularly made of rare earth oxides, preferably yttrium oxide or cerium oxide, is applied to said metallic substrate.
 12. A process according to claim 1 in which a smoothing layer with a thickness of 1 to 50 μm, preferably of 10 to 30 μm, is applied to said metallic substrate.
 13. A process according to claim 12 in which a smoothing layer, consisting of an oxide ceramic substance, particularly a zirconium oxide containing substance, and a stabilizer, particularly made of rare earth oxides, preferably Yttrium oxide or cerium oxide, is applied to said metallic substrate.
 14. A process according to claim 1 in which said metallic substrate is moved by simple rotating or hunting movements in the particle cloud of the plasma jet.
 15. A process according to claim 1 in which the application of said coating system is performed by low pressure plasma spraying according to the thin film method if the individual layers of said coating system are very thin.
 16. A process according to claim 1 in which a coating system is created that comprises, starting from said metallic substrate, the following individual layers in the following sequence: a barrier layer; a hot gas corrosion protection layer made of a MCrAlY-alloy, whereby M is Fe, Co or Ni, or of a metal aluminide a protection layer on an oxide basis for the protection of said hot gas corrosion protection layer; a heat barrier layer on a ceramic basis; and a smoothing layer for improving the erosion resistance.
 17. A process for applying a heat shielding coating system on a metallic substrate consisting of a Ni- or Co-alloy or another highly heat-resistant metallic alloy, said coating system comprising at least three individual layers selected from the following group of layers: Barrier layer; Hot gas corrosion protection layer; Protection layer; Heat barrier layer; Smoothing layer; said coating system being applied to said metallic substrate by low pressure plasma spraying in a single operation cycle.
 18. A process according to claim 18 in which said metallic substrate is a turbine blade, particularly a stator blade or a rotor blade of a stationary gas turbine or of an aircraft jet engine.
 19. A process according to claim 18 in which said metallic substrate is a component of a stationary gas turbine or of an aircraft jet engine that is subjected to hot gas, particularly a heat shield.
 20. Metallic component made of a Ni- or Co-alloy, having a heat shielding coating system comprising at least three individual layers selected from the following group of layers: Barrier layer; Hot gas corrosion protection layer; Protection layer; Heat barrier layer; Smoothing layer; said coating system having been applied to said metallic component by low pressure plasma spraying in a single operation cycle.
 21. Turbine blade, particularly a stator blade or a rotor blade of a stationary gas turbine or of an aircraft jet engine, having a heat shielding coating system comprising at least three individual layers selected from the following group of layers: Barrier layer; Hot gas corrosion protection layer; Protection layer; Heat barrier layer; Smoothing layer; said coating system having been applied to said turbine blade by low pressure plasma spraying in a single operation cycle.
 22. Metallic component of a stationary gas turbine or of an aircraft jet engine that is subjected to hot gas, particularly a heat shield, having a heat shielding coating system comprising at least three individual layers selected from the following group of layers: Barrier layer; Hot gas corrosion protection layer; Protection layer; Heat barrier layer; Smoothing layer; said coating system having been applied to said metallic component by low pressure plasma spraying in a single operation cycle. 